An electronic control unit (hereinafter often referred to as ECU) requires various power source voltages depending upon the applications. Particularly, when a motor is to be driven by a known H-bridge or a three-phase bridge constituted by switching elements, there is usually used a method of boosting a power source voltage supplied to the ECU through a booster circuit as a gate drive source for the MOSFETs (metal oxide semiconductor field-effect transistors) which are the switching elements.
The booster circuit boosts the supplied voltage to be roughly twice as great or more. There, however, exists an upper limit in the voltage (i.e., breakdown voltage) applied to the parts and elements constituting the booster circuit and the electric circuit such as the H-bridge or the three-phase bridge, and voltages in excess of the upper limit shall not be applied thereto. When a high voltage is supplied to the booster circuit, the voltage is further boosted and often exceeds the breakdown voltage causing the circuit to be broken down. So far, therefore, a monitoring function has been provided to monitor the supplied voltage, and the operation of the booster circuit is discontinued in case the supplied voltage exceeds a predetermined value.
There have further been devised a semiconductor memory device which measures the time until the supplied voltage is boosted to a predetermined voltage and operates the booster circuit for the measured period of time only, and a circuit for forming the boosted voltage for the semiconductor memory device (see JP-A-2003-123495).
In many cases, however, it has been demanded to obtain a predetermined boosted voltage despite an increase in the supplied voltage as a result of fluctuation. In an electric power-assisted steering device of a vehicle, for example, an alternator generates an overvoltage at the time of load dumping (in case a battery terminal is disconnected). Therefore, the power source voltage (voltage supplied from the battery) rises abruptly. In case the supplied voltage exceeds a predetermined value in the prior art, the operation of the booster circuit is discontinued to protect the circuit. Therefore, no voltage for driving is supplied to the switching elements of the inverter. When the supplied voltage becomes lower than the predetermined value, the booster circuit is driven again. While the booster circuit is not in operation, therefore, the motor is not driven by the inverter which is for assisting the steering force; i.e., a problem arouses in that the steer-assisting force abruptly decreases and the feeling of steering is deteriorated.
The boosted voltage necessarily decreases when the booster circuit is no longer operated. However, the supplied voltage is an overvoltage generated by the alternator and does not decrease despite the booster circuit no longer being operated. Therefore, the ECU cannot monitor the supplied voltage to control the operation of the booster circuit so as not to interrupt the operation of the electric motor. This holds not only at the time of load dumping but also in the case of the ECU to which a high voltage is supplied at all times from the battery (e.g., when the battery voltage is 42 V).